[WIP][FX] new sections in docs (#50562)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50562
Adding new top-level sections to the docs to be filled out
Test Plan: Imported from OSS
Reviewed By: Chillee
Differential Revision: D25919592
Pulled By: jamesr66a
fbshipit-source-id: 45f564eb8fddc7a42abb5501e160cca0dd0745c8
